The present invention relates generally to accelerometers employing piezoelectric materials and more specifically to shear piezoelectric sensors responsive to acceleration or vibration.
Accelerometers utilizing the shear principle have some special advantages compared to the standard compression type accelerometers as they are considerably less strain sensitive to mounting conditions.
A typical model of an accelerometer widely known on the market is described in Swiss Patent 547 489 and is shown in FIGS. 1 and 2 as prior art. Two shear type piezoplates 3 are screwed to a center post 2 with two masses 4 under the bolt heads.
Besides the requirement of insulation washers 10 and electrodes 9 to separate the signal from the structure, the state of the art systems have other disadvantages. The introduction of clearly defined shear forces to the faces of the piezoplates is not possible since the bolt can introduce forces to the bore of the piezoplates. In addition, the masses might absorb bending forces introduced by the bolts screwed into the center post. Thus the sensitivity of prior art accelerometer depends very much on the amount of the torque applied to the bolts as well as of the assembly procedures.
According to the presented invention, all these uncertainties will be eliminated by providing novel means to exert only shear forces on the piezoplates in the main measurement direction z, independent of the torque of the pre-stress bolt, In addition no insulating washers and electrodes are required to obtain the measuring signal from the piezoplates.
The invention thus provides a new type of shear accelerometer of very simple design that can be produced to very accurate and repetitive specifications as no unwanted torque and force-moment effects can occur. Due to its design and production simplicity, the invention can be produced at a very competitive price level.
A further object of the invention is the symmetric distribution of clamping forces of the entire surfaces of the piezoplates and most important is the fact that the vectors (V) of clamping forces are perfectly perpendicular to the piezoplates and parallel to the axis of the through-bolt arrangement. These measures allow an absolute intimate contact of the shear transmitting faces with the result that an outstanding frequency response can be expected.
A further object of the invention is the possibility to include one more mass-screw arrangement to obtain a two or a three axis accelerometer all to the same design principle.
These and other objects are achieved by securing the piezoplates and the masses, each having a bore therein aligned with a bore of a center post of the body of a accelerometer, by a bolt extending through the bores to clamp the mass and piezoplates to the center post without contacting the center post. This provides mechanical and electrical isolation of the bolt from the center post. The bores are aided in their alignment by their common external dimensions on at least two sides and the bolt is aligned to the bores. The bolt may be mechanically and electrically isolated from the center post and the piezoplates by an annulus void or the annulus void may be filled with an electrically insulating and non-force transmitting sleeve.
In a symmetrical accelerometer arrangement, the pair of piezoplates are separated by the center post and the pair of masses are separated by the piezoplates, thus providing one axis sensing. In another embodiment, a first pair of piezoplates are separated from a second pair of piezoplates by the center post and a pair of masses are separated by the first and second pair of piezoplates, thus providing two axis sensing. A single bolt secures both pairs of piezoplates to the center post.
A combination of these two may be used for three axis of sensing wherein a pair of opposed piezoplates which are separated by the center post is secured by one bolt in a first bore and the two opposed pairs which are separated by the center post are secured by a second bolt in a second bore orthogonal to the first bore. Each piezoplate of the two pairs of piezoplates on the common bolt have a thickness, along the axis of the bore, half the thickness of the single pair of piezoplates along its bore. The single bolt pair of piezoelectric devices are shear type piezoplates and the two pairs along a single bolt include one pair of shear type piezoplates and a pair of compression type piezoplates or two pairs of shear type piezoplates. This accelerometer arrangement provides sensing forces along the x, y and z axis. By selecting the polarity of the piezoplates of a pair, linear or angular acceleration is sensed.
The orthogonal first and second bores may be in a single center post spaced vertically along the center post or may be in a pair of center post orthogonal to each other. The orthogonal center post would have generally an L or T shape.
To provide adjustment of the axis of sensitivity of the piezoelectric plates, the housing includes two orthogonal mounting bores in a plane perpendicular to the center post. One of the mounting bores is co-planar with the bore which receives the bolt mounting the pair of piezoelectric plates to the center post. The piezoelectric plates mounted to the center post in this embodiment may have parallel axis of sensitivity or orthogonal axis of sensitivity.
The external dimension on at least three sides of the center post, piezoplates and the mass are similar to aid alignment of the bores. An alignment tool having internal dimensions similar to the external dimensions aligns the bores during tightening of the bolts.